The most important property of the separator used for various batteries is electrolyte retainability. If the separator is low in electrolyte retainability, it causes problems that it increases the internal resistance of the electrochemical device, resulting in capacity decrease, voltage down and short working life thereof.
For example, Japanese Patent Application Laid-Open Publication No. 3-105851 (Patent Literature 1) discloses, “A separator for a battery such as a lithium primary or secondary battery, which separator is a microporous membrane having a weight-average molecular weight/number average molecular weight of 10 to 300, and having a thickness of 0.1 to 25 μm, a porosity of 40 to 95%, an average through-bore diameter of 0.001 to 0.1 μm, a width of 10 mm and a rupture strength of 0.5 kg or more, formed of a polyethylene composition containing 1 percent by mass or more of an ultra-high molecular weight polyethylene having a weight-average molecular weight of 7×105 or greater”.
However, this type of separator has an extremely small pore size that is on the order of submicron or smaller and thus when the viscosity of an electrolyte is high, has had a problem that the electrolyte is unlikely to permeate into the separator and the efficiency of the battery assembly becomes poor. Furthermore, since the separator has the fine pores formed linearly in the thickness direction and thus is rather poor in electrolyte retainability, the expansion and contraction of the electrode progresses accompanied with repeated charge and discharge causing a problem that the separator is compressed to push out the electrolyte retained therein and thus gradually reduced in capacity.
Recently, the use of a non-woven fabric having excellent liquid retainability and the like as a separator has been, therefore, proposed. The non-woven fabric has an advantage of excellent electrolyte retainability because it is formed of fibers each stacked relatively randomly in the thickness direction and thus the fine pores are not formed linearly.
However, if the conventional non-woven fabric is made too thin, it may cause a cathode and an anode to short-circuit. Whereas, the fabric is made thick, it can prevent a cathode and an anode from short-circuiting but has a drawback that the energy density is decreased. Therefore, the non-woven fabric is inadequate for the use in a lithium-ion secondary battery.